In order to obtain a frequency response of a resonator filter, which meets the specifications, it is necessary to have right coupling strengths between the resonators, and a certain resonator frequency, or natural frequency for each resonator. In series production the variation of the natural frequencies of resonators made in the same way is usually too large to keep all natural frequencies at a sufficiently right value. Therefore each resonator in each filter must be tuned individually. Here such tuning is called basic tuning. If the filter is intended to be used as a part in a system, where the transmitting and receiving bands are divided into sub-bands, then the width of the passband of the filter must equal the width of a sub-band. Further the passband of the filter must be located at the desired sub-band. An adjusting of the natural frequencies of the resonators is sufficient to shift the passband; it is not necessary to change the couplings between the resonators.
Previously it is known the use of tuning screws in the adjusting of the natural frequency of a resonator. The lid of e.g. coaxial resonator is equipped with a metal screw at the inner conductor of the resonator. When the screw is turned, the capacitance between the inner conductor and the lid changes, in which case also the natural frequency of the resonator changes. A disadvantage in the use of tuning screws is that in a multi-resonator filter it may be necessary to manually turn the screws in many stages to obtain the desired frequency response. Thus the tuning is time consuming and relatively expensive. The screw accessories increase the number of components in the filter, and the threaded screw holes mean an increased number of work steps. These facts will raise the manufacturing costs on their part. In addition the electrical contact in the threads may deteriorate in the course of time, which results in tuning drift and in increased losses in the resonator. Moreover, there is a risk of electric breakdown in high-power filters of the transmitting end if the point of the screw is close to the end of the inner conductor.
Regarding a coaxial resonator, the capacitance between its inner conductor and the surrounding conductive parts can be changed by means of bendable elements, too. In a known structure there is a planar extension at the end of the inner conductor, in parallel with the lid. At the edges of the extension there is at least one projection parallel with a side wall, which functions as a tuning element. By bending the tuning element said capacitance and, at the same time, the natural frequency of the resonator will be changed. A disadvantage of that kind of solution is that in a multi-resonator filter it can be necessary to manually bend the tuning elements in several stages to obtain the desired frequency response. The filter's lid must be opened and closed for each tuning stage. Thus also in this case the tuning is time-consuming and relatively expensive. This is emphasized by the use of sub-bands, as the filters must be tuned for each sub-band during the manufacturing.
FIG. 1 presents a tuning way of resonator filter using a dielectric tuning element, the way being known from the publication JP 62123801. In the figure there is a longitudinal section of one coaxial resonator 110 of the filter, the resonator comprising a bottom 111, an inner conductor 112–113, an outer conductor 114 and a lid 115. The outer conductor surrounds the inner conductor over its whole length, as normally. In addition, the resonator comprises in this example a cylindrical dielectric block having an axial, vertical hole. The block has been coated by conductive material apart from its upper surface. The inner conductor consists of that coating material 112 and a cylindrical conductor 113 having a firm contact with the wall of the hole. The widened upper end of the cylindrical conductor extends above the upper surface of the dielectric block. The bottom 111 shorts the transmission line formed by the inner and outer conductors at its lower end. At the upper end of the structure the inner conductor does not extend to the conductive lid, so the transmission line is open at the top. A result of this is that the structure functions as a quarter wave resonator.
For the tuning of the resonator 110 there is a conductive screw 117 in its lid 115. A cylindrical dielectric tuning element 118 has been attached on the lower surface of the screw, the tuning element being made of material, which has relatively high dielectricity, such as ceramics. That dielectric tuning element is located in the resonator cavity above the inner conductor of the resonator, at certain distance d from the upper surface of the inner conductor. When the screw 117 is turned deeper, for instance, the distance d is decreased. In that case the effective dielectricity between the inner conductor and the lid increases, because the ceramics fills greater part of the space therebetween, in the proportion. The capacitance between the inner conductor 116 and the lid is increased for the increase of the dielectricity and for the approach of the conductive screw, on the other hand. The increase of the capacitance results in increase of the resonator capacitance results in increase of the resonator electric length and lowering in the resonator natural frequency. Disadvantages of this solution, too, are that in a multi-resonator filter it may be necessary to manually turn the screws in many stages to obtain the desired frequency response, and that the electrical contact in the screw joint may deteriorate in the course of time.
FIGS. 2a,b present another example of a known resonator filter, for tuning of which a dielectric tuning element is used. In FIG. 2a there is a longitudinal section of the filter 200 and in FIG. 2b it is seen from above the lid 205 cut open. The filter comprises a conductive housing formed by a bottom, outer walls and the lid, the space of the housing being divided into the resonator cavities by conductive partition walls. In each cavity there is, to reduce the resonator size, a fixed cylindrical dielectric block, such as the dielectric block 216 of the first resonator, visible in the figure. The bases of the cylinder are parallel with the bottom and the lid of the resonator, and the block has been raised over the resonator bottom 211 by a dielectric support piece SU. The support piece has substantially lower dielectricity than the dielectric block 216. The dielectric block has been dimensioned so that a transverse electric wave TE01 is excited in it at the operating frequencies of the filter. The resonators then are half wave cavity resonators by type.
For the tuning of the first resonator of the filter 200, in the resonator's lid there is a screw 217 made of a dielectric material such as a plastic. A cylindrical, e.g. ceramic tuning element 218 has been attached on the lower surface of the screw. That tuning element is located in the resonator cavity above the dielectric block 216, at certain distance from the upper surface of the block. When the screw 217 is turned e.g. deeper the tuning element 218 approaches the dielectric block 216. In that case the electric size of the dielectric block increases, and the natural frequency of the block and the whole resonator lowers. Disadvantages of this solution, too, are that in a multi-resonator filter it may be necessary to manually turn the screws in many stages to obtain the desired frequency response.